Acute allergic reaction (anaphylaxis) in dogs and cats
Severe allergic reactions can kill within minutes — head straight to the emergency vet at the first sign of facial swelling, hives, vomiting after a sting/injection, or breathing difficulty.
Recognise the signs
- Mild reaction: facial swelling, hives (raised lumps under fur), itching, mild vomiting
- Moderate reaction: marked muzzle/eye swelling, persistent vomiting, diarrhoea
- Severe reaction (anaphylaxis): collapse, weakness, pale or blue gums, difficulty breathing, sudden severe vomiting or diarrhoea (often bloody in dogs), shock
- Onset typically within 5–30 minutes of trigger but can be immediate
First aid steps
- Phone the emergency vet and start driving — do not delay to take photos or wait it out.
- If the trigger is visible (insect sting visible) and easy to remove, scrape it out with a card edge.
- Keep the pet calm and minimise movement.
- If the pet collapses and stops breathing en route, start CPR.
- Note the time of trigger exposure and when symptoms started — the vet will need this.
Do NOT
- Do not give human EpiPens unless explicitly trained and told to by a vet — dose and route differ for animals.
- Do not give human antihistamines without vet advice — formulation matters and dosing is different.
- Do not wait to see if mild swelling resolves if it is progressing.
- Do not assume past mild reactions mean the next will be mild — anaphylaxis can develop on subsequent exposures.
While transporting to the vet
- Lay the pet on its side with head extended.
- Loosen collars.
- Keep warm but do not overheat.
- Phone ahead so adrenaline, fluids and oxygen are ready.
When to phone the vet immediately
- Any swelling around the face, eyes, or throat
- Difficulty breathing or noisy breathing
- Pale, blue, or muddy gums
- Collapse or weakness
- Severe vomiting or diarrhoea after a sting, vaccine, or new food
- Hives covering large areas of the body
Signs that can usually wait for a routine appointment
- Localised mild swelling at a sting site, with the pet bright, eating, breathing normally and gums staying pink, can be monitored at home for 60 minutes — but escalate at the first sign of progression.
Common causes
- Insect stings — bees, wasps, hornets
- Vaccinations or injectable medications (uncommon but recognised)
- Foods (less commonly truly anaphylactic, more often skin and gut signs)
- Snake bites (UK adders)
- Spot-on flea treatments — particularly permethrin in cats
- Plants and contact allergens
What the vet will need to know
- Suspected trigger — sting, food, medication, vaccine
- Time of exposure and time of first symptoms
- Progression of symptoms (improving, stable, worsening)
- Any previous allergic reactions and how managed
- Current medications
- Pet's weight
Aftercare
- Most cases respond well to adrenaline, IV fluids, steroids, and antihistamines.
- Hospitalisation 12–48 hours for monitoring — biphasic reactions can recur 6–12 hours later.
- Some pets are sent home with an emergency plan and prescribed antihistamines.
- Discuss avoidance and an emergency action plan for future exposures.
Prevention
- Identify and avoid the trigger where possible.
- Note reactions in vet records — vaccines may be staggered or pre-medicated next time.
- Never use dog spot-on products on cats.
- Keep antihistamines (vet-recommended brand and dose) at home for known mild reactors.
- For severe known reactors, ask the vet about a written emergency plan and possible adrenaline auto-injector training (rare in vet medicine but discussed in some cases).
Breed-specific notes
- Vaccine reactions slightly more common in small breeds (Dachshund, Boston Terrier, Chihuahua).
- Cats are highly sensitive to permethrin — never use dog flea spot-ons on cats.
Frequently asked questions
Can I give my dog Piriton for an allergic reaction?
Chlorphenamine (Piriton) is sometimes used in dogs at vet-directed doses for mild reactions, but it is not enough on its own for anaphylaxis. For any rapidly progressing reaction, head to the vet — antihistamines do not reverse the airway swelling and circulatory collapse of true anaphylaxis.
How quickly does anaphylaxis happen?
Most reactions appear within 5–30 minutes of exposure. Some are essentially immediate (within seconds), and a smaller proportion show a 'biphasic' second wave 6–12 hours after the first response — which is why hospitalisation overnight is common after a severe reaction.
Is it safe to vaccinate a pet that has had a reaction before?
Often yes, with adjustments — splitting vaccines across visits, pre-medicating with antihistamines, or omitting non-essential vaccines. Always tell the vet about any past reaction so the protocol can be adapted.